As is well known in the art, significant advances have been made in telecommunications systems over recent years, particularly in the rate at which information can be communicated. Modern digital telecommunications systems and communication media provide very high bandwidth, such as the 44.736 Mbps data rate provided by the DS-3 data frame standard. Furthermore, conventional fiber optic cable and systems can provide even higher bandwidth and data rates by time-division multiplexing of up to twelve DS-3 lines, providing bandwidth of up to 536.8 Mbps.
These extremely high bandwidths now available in digital telecommunications systems have enabled the communication of large volumes of data at high speeds. Since voice channels require very little bandwidth (on the order of 4 kbps each), a large number of voice channels may now be communicated over a single communication line by way of time division multiplexing. The available bandwidth now also enables the communication of large blocks of digital data from computer-to-computer, as well as digital data representative of other media such as video displays.
Unlike voice transmissions, however, in which some amount of errored signals can be readily tolerated without garbling of the message, the successful transmission of digital data among computers requires high reliability and high quality transmission. Accordingly, conventional digital cross-connects now provide "performance monitoring" (commonly referred to as "PM"), by which the error rate of received digital data is monitored by way of cyclic redundancy check (CRC) and other conventional coding techniques. Such performance monitoring is used to ensure the desired grade of service desired by those telecommunications customers paying premium tariffs for high quality and low error rate communications.
Conventional telecommunications systems also generally provide some amount of redundancy so that failure of a telecommunications line or network element does not result in the loss of the communicated message. Conventional telecommunications systems with performance monitoring have implemented certain alarm conditions by which a human operator is alerted to events such as "loss of signal" and to error rates exceeding various thresholds. In response, the operator can manually switch to a redundant line to again enable communication of the digital data in the system. Of course, the procedure of generating an alarm condition and the manual switching of input/output ports to other lines cannot be effected quickly.
By way of further background, conventional fiber optic terminals (commonly referred to as FOTs) have implemented 1:1 redundancy for the fiber optic lines in a system, with some amount of automatic switching. According to this 1:1 redundancy scheme, the overhead portion of the bandwidth is monitored to determine if a loss-of-signal ("LOS") or alarm indication signal ("AIS") condition is being received. In these FOT 1:1 redundancy schemes, upon receipt of an LOS or AIS signal, the FOT will automatically switch its transmission to the other of the two fiber optic lines, enabling transmission of the data despite the failure of the first fiber optic line.
FIG. 1 illustrates the bandwidth in a conventional 1:1 redundant fiber optic lines 1E, 1W, as received by a conventional fiber optic terminal (FOT) 3. Each of fiber optic lines 1E, 1W in this example are of the OC-12 type, and as such bidirectionally communicate twelve DS-3 lines in time-domain multiplexed fashion. The DS-3 paths occupy much of the available bandwidth, as shown in FIG. 1. According to conventional standards, a portion of the remainder of the bandwidth communicated by fiber optic lines 1E, 1W is reserved for line data, such as framing and synchronization signals, frame identifying signals, and also signals such as LOS and AIS which indicate a line failure somewhere in the system. This line data portion of the bandwidth is also referred to as overhead, as it does not carry any traffic data. Fiber optic terminal 3 is operable to multiplex and de-multiplex the data to and from one of fiber optic cables 1E, 1W, for communication with twelve individual DS-3 lines.
According to the conventional 1:1 redundancy scheme, conventional FOT 3 monitors the line data and switches communication between fiber optic lines 1E, 1W upon receipt of a signal indicating a line failure. However, many conditions other than the LOS and AIS conditions are unacceptable to the telecommunication customer that is demanding high quality communication, particularly where computer data is to be transmitted and received. These other conditions may not be of such degree as to cause loss of an entire line (i.e., all channels or "paths" being communicated along the fiber optic line), but may be due to a hardware failure for a single path within the line. Examples of these other conditions include loss-of-frame ("LOF"), and "soft" errors in which a particular path has experienced a high rate of errored seconds. The mere monitoring of line data by conventional FOT 3 provides no visibility into these other path-related conditions, and as such the redundancy scheme of FIG. 1 is unable to provide the high level of protection desired by certain customers. Conventional FOT 3 thus only provides line protection, and as such requires doubling of the available fiber optic capacity from that required for traffic only, to accomplish such line protection. As such, the utilization of system fiber optic cable is relatively low in such situations.
It is therefore an object of the invention to provide a method and system for providing facility protection for a digital telecommunications system, which monitors path data and provides automatic switching on both hard and soft error conditions.
It is a further object of the present invention to provide such a method and system to provide such protection where the switching time between the primary and secondary lines for a failing path is extremely short, such as on the order of 50 msec or less.
It is a further object of the present invention to provide such a method and system which may be readily implemented into existing cross-connect installations.
It is a further object of the present invention to provide such a method and system in which the possibility of blocking is much reduced.
It is a further object of the present invention to provide such a method and system in a distributed manner in a digital cross-connect.
It is a further object of the present invention to provide such a method and system in which the soft error rate thresholds may be modified.
It is a further object of the present invention to provide such a method and system in which the utilization of fiber optic cables may be increased dramatically.
It is a further object of the present invention to provide such a method and system in which transmission media of different types (e.g., microwave and light wave) may provide protection for one another.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.